# High-Frequency RFID Tags: An Analytical and Numerical Approach for

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```					       Analytic Expressions for Small Loop
Antennas-With Application to EMC and
RFID Systems
Yuxin Feng                           Benjamin D. Braaten
Electrical and Computer Engineering       Electrical and Computer Engineering
North Dakota State University             North Dakota State University
Fargo, North Dakota 58105                 Fargo, North Dakota 58105
Email: yuxin.feng@ndsu.edu                Email: benbraaten@ieee.org

Robert M. Nelson
Electrical and Computer Engineering
North Dakota State University
Fargo, North Dakota 58105
Email: r.m.nelson@ieee.org
Topics
•   Introduction.
•   Derivation of analytical expressions.
•   Verification of analytical expressions.
•   Results
– Induced currents on a loop from incident field
– Induced currents on a loop from another loop.
• Conclusion.

2
Introduction
• A new way of thinking about the operation of
inductively coupled RFID systems
• Loops are used in may applications
– “sniffer” probes
– Inductively couple energy to implantable devices [1]-
[5]
– Wireless communication
– RFID [6]
• Readertransponder(tag) via EM waves.
• Harvests energy and communicates.

3
Introduction
Low frequency RFID Tag

4
Introduction
• We investigate the induced current in a
small loop as well as the magnetic field
created by that induced current as well as
• Two sources
– Incident field
– Other small loop
• Verified with MININEC [7]
5
Derivation of analytical expressions

Consider the following
receiving antenna
equivalent circuit [8]-[9]:

When the radius of the loop is less than .03λ [9] we can
assume the current is constant in the loop. This then
gives:
(1)
6
Derivation of analytical expressions
The voltage developed across the open circuit
terminals of a loop antenna is related via
Faraday’s law to a time-varying magnetic flux
cutting through the loop [8]-[10].
For an N-turn loop Faraday’s law gives the
phasor-domain voltage:

(2)
where is the radian frequency, is the
magnetic flux density, and is the incremental
7
surface area of the loop.
Derivation of analytical expressions
Since we assumed a small loop size we can
approximate the magnetic flux crossing the loop as:

(3)
where         and    is the radius of the loop. This then
gives:
(4)
where            .
If desired, the voltage across the load ZL can be
obtained from Voc as
(5)
8
Derivation of analytical expressions
To determine the current induced in the loop we need to
know the input impedance of the loop antenna. In
general this impedance is expressed as:
(6)

where     and     are the input resistance and reactance of
the antenna, respectively.
The input resistance is typically modeled as having two
components,                                          (7)

where      is the radiation resistance of the antenna and
is the ohmic (or loss) resistance of the antenna.
9
Derivation of analytical expressions
The radiation resistance of a small, N-turn loop is
approximately [8]:

(8)

and the ohmic loss for each turn can be accounted for by

(9)

for a wire with conductivity   , loop radius b, and wire
10
Derivation of analytical expressions
Small loops tend to be very inductive, with
inductance of an N-turn loop typically
approximated as [12]

(10)

The reactance of a small loop tends to be
larger [13]. This assumption is made in
this work.
11
Derivation of analytical expressions
This then gives the input impedance of a N-
turn small loop as approximately:

(11)

Using (11) and the expression for the open
circuit voltage in (4) we can express the
induced current in a small N-turn loop as:

Notice
(12)   12
Derivation of analytical expressions
If the current I flowing in a small loop of radius b centered at
the origin in the x-y plane is known the electric and
magnetic fields created by the current can be determined
by the following expressions [8]-[11]:

(13)

(14)

(15)
where    is the intrinsic impedance of the medium
through which the wave is propagating.                          13
Derivation of analytical expressions
• First we will consider the situation when the loop
antenna is far from the source then we will
consider the situation when the incident
magnetic field is another loop lying in close
proximity of the receiving loop.
• In an effort to think of RFID systems from an
induced-current/scattered-field perspective we
determine the induced current and scattered
field for two different load impedances.
• It will be clearly shown that the “reader” can
detect what happens at the “tag” when the load
of the tag changes.
14
Verification of analytical
expressions
Consider the small single-turn
loop with a loop radius of 1mm.
This loop was modeled in
MININEC with 50 segments.
For each segment the wire
radius was .1mm. A 1A current
source was used to excite the
problem.

The following is the magnitude
and angle of the input
impedance.
15
Verification of analytical
expressions
30

20
|Zin| ( )

10
MININEC
Analytical
0
0   1   2   3   4     5     6    7   8       9        10
8
x 10
91
Phase (Degree)

90.5

90

89.5                                             MININEC
Analytical
89
0   1   2   3   4     5      6   7   8       9        10
Frequency (Hz)                        8
x 10

16
Verification of analytical
expressions
• Calculation of the input impedance with MININEC verified
that the assumption of constant current distribution used in
deriving the expression for the input impedance of a loop
is fairly reasonable for the given simulation frequency
range (10 MHz to 1 GHz).
• To determine the effect the loop size has on the current, a
loop with a loop radius of 2.5cm was evaluated in
MININEC
• The wire radius was 0.1 mm and the loop was driven with
1 A current source.
• The simulation was carried out for three frequencies (13
MHz, 300 MHz and 1 GHz).
17
Verification of analytical
expressions

18
Verification of analytical
expressions
• At 13 MHz b = 0.001λ the current
distribution is essentially constant around
the loop.
• At 300 MHz b = 0.025λ and the current
varies slightly.
• At 1 GHz b = 0.083λ and significant
variation is noted.

19
Results
• Consider the same loop described above – i.e., a loop of
1 mm radius made from 1 turn of 0.1 mm wire – to
investigate the induced current on a small loop as a
result of an incident magnetic field.
• The intent of this work is to provide approximate closed-
form expressions that can be used to describe the
operation of inductive RFID systems from an induced-
current/scatted-field perspective.
• As a check on accuracy, the results of our analytical
expression for current is compared with those
determined from MININEC when the incident magnetic
field is 1 A/m.

20
Results
2
1.5
|Iind| (mA)

1
0.5           Analytical

0
0   1   2      3     4     5     6      7        8   9      10
8
x 10
200

Phase (Degree)

100            MININEC
Analytical
0

-100

-200
0   1   2      3     4     5      6     7        8   9      10
Frequency (Hz)                         8
x 10

21
Results
• Next we investigate the effect the previous
change in current has on the scattered
field.
• This is done by determining the scattered
field for two locations, as shown in the
following figure.

22
Results
•Position (1) models what scattered field might be
expected in a “reader loop” located in the same plane as
the “tag loop”.
•Position (2) models what might be detected in a “reader
loop” placed directly above or below the “tag loop”.

23
Results
Point (1) (same plane as loop)
1.5
|H| (A/m)

1

0.5

0
0   1      2        3   4     5     6    7   8       9       10
8
x 10
-50
Phase (Degree)

-100

-150

-200
0   1      2        3   4     5      6   7   8       9       10
Frequency (Hz)                       8
x 10

24
Results
Point (2) (directly above loop)
2
|Hr| (A/m)

1

0.5

0
0   1      2        3   4     5     6    7   8   9      10
8
x 10

200
Phase (Degree)

0

-100

-200
0   1      2        3   4     5      6   7   8   9      10
Frequency (Hz)                  8
x 10

25
Results
• We are also interested in observing how the
scattered field varies with distance from the loop.
This is illustrated in the following figure, where
the scattered field above the “tag loop” is
determined when the operating frequency is
fixed at 500 MHz.
• This provides the important result that the ability
to remotely distinguish what happens at a “tag”
diminishes as one moves away from the tag.

26
Results
1.5
|Hr| (A/m)

1

0.5

0
0.1      0.2    0.3     0.4      0.5     0.6       0.7     0.8     0.9       1

200
Phase (Degree)

100

0

-200
0   0.1   0.2    0.3     0.4     0.5    0.6     0.7    0.8     0.9      1
Distance (m)

27
Results
• Now suppose one has a reader that is
inductively coupled with the tag.
• Again assume both loops are small enough such
that a constant current exists.
• This then allows us to determine the induced
current on the tag antenna as well as the
induced current on the reader antenna as a
result of the scattered field from the tag.

28
Results
• This method can be used for any loop position.
• In this (initial) work we provide the results for
one particular case.
• The scenario under investigation is shown below

29
Results
Assuming that the original current in the
reader is I0 and the loops have center-to-
center spacing distance d then the
scattered field back at the transmitter (i.e.,
reader) can be written as:

(16)

30
Results
This then results in an induced current at the

(17)

31
Results
• As a final numerical result, assume that
the input impedance of the reader is 50Ω,
I0=1A, d=10 mm, and the two loops are in
the same plane.
• This then gives the following induced
current at the reader as a results of the
scattered fields from the tag.

32
Results

33
Conclusion
• We investigated the current induced in a small
loop as a results of
– An incident magnetic field
– Another small loop in close proximity
• We illustrated how the load on the tag antenna
can effect the scattered field and hence the
induced current on the reader
• Alternate way of thinking about RFID systems
• Useful for EMC engineers for estimating currents
induced on small loops in RFID systems

34
Acknowledgements
• Defense Microelectronics Activity (DMEA)
• Mr. Aaron Reinholtz of the Center for
Nanoscale Science and Engineering
(CNSE)
• Dr. Greg McCarthy of the Center for
Nanoscale Science and Engineering
(CNSE)

35
References
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209, 1999.
References
[5]    B.J. Feder and T. Zeller, “Identity badge
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Jan. 5, 2005,
http://copland.udel.edu/stuorg/mds/pelumisar
approvedforuseinhealthcare.pdf/
[6] K. Finkenzeller, RFID Handbook, John Wiley
and Sons, West Sussex, England 2003.
[7] J.W. Rockway and J.C. Logan, MININEC
Broadcast Professional for Windows, EM
Scientific, Inc. Carson City, NV, 1996.
37
References
[8]  W. Stutzman and G. Thiele, Antenna
Theory and Design, New York, NY:
Wiley, 1998.
[9] Constantine A. Balanis, Antenna
Theory Analysis and Design, Harper
and Row, New York, 1982.
[10] C.R. Paul, Introduction to
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38
References
[11] W.H. Hayt and J.A. Buck, Engineering
Electromagnetics, 7th ed., New York, NY:
Mc-Graw Hill, 2006.
[12] C.S. Walker, Capacitance, Inductance and
Crosstalk Analysis, Norwood, MA: Artech
House, 1990.
[13] Boeshans, D. Farden and R. Nelson,
“Design, testing and computer modeling of a
tunable loop antenna,” in Proc. of the 2002
IEEE Int. Sym. on Electromagnetic
Compatibility, Minneapolis, MN, August
2002, pp. 705-710.
39

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